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The field of the invention is motor controllers and more specifically field oriented controllers and a method and apparatus for identifying a rated flux current estimate for an electromagnetic machine during a static commissioning procedure.
A typical three-phase induction motor controller is provided with three phases of electrical voltage and uses the three phases to produces a rotating magnetic stator field within the stator cavity of a motor linked thereto. The stator field induces (hence the label xe2x80x9cinductionxe2x80x9d) a rotor current within a rotor mounted in the stator cavity. The rotor current in turn generates a rotor field within the cavity. The rotor field interacts with the stator field (e.g., is attracted to or repelled by) and causes the rotor to rotate.
The magnitude of the attractive force between the rotor and stator fields is generally referred to as torque. Where the force between the two fields is high, the torque is high and the force that can be applied to a load is high. Where the attractive force between the stator and rotor fields is low, the torque is low and the force that can be applied to a load is also relatively low.
To a first approximation, the torque and speed of an induction motor may be controlled by changing the frequency of the driving voltage and thus the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the speed of the rotor (which follows the stator field). Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip, that is, the difference in speed between the rotor and the stator field. An increase in slip increases the rate at which flux lines are cut by the rotor, increasing the rotor-generated field and thus the force or torque between the rotor and stator fields.
Referring to FIG. 1, the rotating phasor 14 of a stator magneto motive force (xe2x80x9cmmfxe2x80x9d) will generally form some angle a with respect to the phasor of rotor flux 18. The torque generated by the motor is proportional to the magnitudes of these phasors 14 and 18 but is also a function of the angle xcex1 between the two phasors 14 and 18. The maximum torque is produced when phasors 14 and 18 are at right angles to each other (e.g., xcex1=90xc2x0) whereas zero torque is produced when phasors 14 and 18 are aligned (e.g., xcex1=0xc2x0). The mmf phasor 14 can be usefully decomposed into a torque producing component 15 perpendicular to the phasor 18 and a flux component 17 parallel to rotor flux phasor 18.
Components 15 and 17 of the stator mmf are proportional, respectively, to two stator currents iqe, a torque producing current, and ide, a flux producing current, which may be represented by orthogonal vectors in the rotating frame of reference (synchronous frame of reference) of the stator flux having slowly varying magnitudes. The subscript xe2x80x9cexe2x80x9d is used to indicate that a particular quantity is in the rotating frame of stator flux.
Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage (hence the speed of the rotation of the stator flux phasor 14) but also the phase of the applied voltage relative to the current flow and hence the division of the currents through the stator windings into the iqe and ide components. Control strategies that attempt to independently control the currents iqe and ide are generally referred to as field oriented control strategies (xe2x80x9cFOCxe2x80x9d).
Generally, it is desirable to design field-oriented controllers that are capable of driving motors of many different designs and varying sizes. Such versatility cuts down on research, development, and manufacturing costs and also provides easily serviceable controllers.
While multi-purpose controllers have reduced manufacturing costs, unfortunately versatile controllers have complicated commissioning processes required to set up a controller to control a motor. Specifically, to control a motor most efficiently, the controller has to be programmed with certain motor unique operating parameters. Because manufacturers of multi-purpose controllers cannot know the specific operating parameters of the motor with which their controllers will be used, the manufacturers cannot set the parameters for the end usersxe2x80x94the users have to set the parameters themselves.
After an electromechanical machine (e.g., a motor) has been manufactured, the machine is typically characterized by several maximum recommended or most efficient operating characteristics (e.g., rated operating current value, a rated voltage value, a rated rotational speed, a rated horsepower, etc.) that are determinable through various tests and procedures. These rated values are determined by manufacturers and are usually provided to end users so that the users can match machine capabilities with applications (e.g., expected loads, speeds, currents, voltages, etc.). Many of these rated values can also be used to commission a motor controller to control the associated motor.
Other operating characteristics cannot be determined until after a motor is linked to a load and, thereafter, are identified by performing some commissioning procedure. For example, a stator resistance rs and a leakage inductance L"sgr" are determinable via various commissioning procedures.
One other operating parameter that is necessary for efficient and effective FOC is the rated flux or d-axis current value (and related q-axis current value) which depends in part on specific motor design and other operating parameters and hence cannot be provided by a controller manufacturer. To identify a rated flux current value, commissioning procedures have been developed that require rotation of the motor rotor while different current levels are injected into the motor windings so that a flux saturation curve can be generated. In some applications rotor rotation prior to motor operation is unacceptable.
Where rotor rotation prior to operation is unacceptable, some processes have been devised for estimating a saturation curve while the motor is at stand still. Unfortunately, the commissioning processes that are used to generate saturation curves while a motor rotor is stationary are not very accurate and the end result is typically poor motor starting performance.
Thus, there is a need for a process whereby a relatively accurate rated flux estimate can be identified during a static commissioning procedure (i.e., prior to motor rotation/operation).
It has been recognized that several of the rated motor operating parameters that are typically provided by motor manufacturers and several other operating parameters that can be derived during static commissioning procedures can be used in an iterative fashion to identify a relatively accurate flux current estimate for use in starting a motor from standstill. More specifically, a stator resistance value rs and a leakage inductance value L"sgr" can be identified using stationary commissioning procedures. Thereafter, a motor torque current (i.e., a q-axis current) can be assumed and used along with the stator resistance rs and leakage inductance L"sgr" values and rated motor voltage, rated current and rated speed to identify a flux value aligned with the d-axis. Next, the flux value and a set of the other parameters identified above can be mathematically combined to generate a torque estimate. Continuing, a rated motor speed and rated horse power can be used to identify a motor rated torque value. The torque estimate is compared to the rated torque estimate and the q-axis motor torque current assumption is altered as a function of the difference between the estimated and rated torques.
The process described above is repeated until the torque estimate is within a tolerable range of the rated torque value. Once the torque estimate is within the tolerable range of the rated torque value, the d and q-axis current values arc stored as rated flux and torque current values. In at least some embodiments convergence on the rated torque value expedited by altering the q-axis torque current assumption (i.e., the q-axis current value) as a function of the magnitude of the difference between the torque estimate and the rated torque value.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.